Conventionally, as such an optical disc device, the one disclosed in Patent document 1 has been known, for example. Based on this precedent with a part thereof being modified, the following description is made with reference to FIGS. 8-10.
FIG. 8(a) is a side view illustrating an optical disc device based on the conventional technology, FIG. 8(b) is a diagram illustrating a grating pattern formed on a grating surface used in the optical disc device, as well as the distribution of light on the grating surface, and FIG. 8(c) is a diagram illustrating the configuration of a signal plane of an optical disc and the distribution of light on the signal plane.
As shown in FIG. 8, upon successively passing through a transparent substrate 3 and the split plane 4a of a polarizing beam splitter 4, laser light 2 emitted from a radiation light source 1, such as a semiconductor laser, etc., is collected by a collimating lens 5 and turns into parallel light. After conversion from linearly-polarized light (P-waves) into circularly-polarized light by a quarter wave plate 6, the parallel light is collected by an objective lens 7 and focused on the signal plane 8a of an optical disc 8 (to form an light spot). Guide grooves 8g, which extend in the direction of rotation of the optical disc 8 (hereinafter referred to as the “optical disc rotation direction”), are formed at constant pitch in the radial direction of the optical disc 8 (hereinafter referred to as the “optical disc radial direction”) on the signal plane 8a of the optical disc 8. Light reflected by the signal plane 8a of the optical disc 8 passes through the objective lens 7 and is converted into linearly-polarized light (S-waves) by the quarter wave plate 6, whereupon it passes through the collimating lens 5 and turns into converging light. After reflection from the split plane 4a of the polarizing beam splitter 4, the converging light passes through a cylindrical lens 9 arranged such that the central axis of the cylindrical surface is inclined at an angle of 45 degrees relative to a plane parallel to the surface of the paper and is incident upon a light detection surface 10a on a light detection substrate 10 located in the vicinity of the circle of least confusion (at the middle of the vertical focal line and horizontal focal line).
There is a rectilinear grating 3b and a rectilinear grating 3c formed on the surface (grating surface 3a) of the transparent substrate 3, with the axis 3Y, which corresponds to the optical disc rotation direction, serving as a boundary therebetween. The shape of the light spot formed on the grating surface 3a by light emitted from the radiation light source 1 and transmitted through the transparent substrate 3 (transmitted light) is a circle 2a, whose center is at the center 30 of the grating surface 3a. The orientation of the respective gratings is perpendicular to the axis 3Y, with the grating phases of the rectilinear grating 3b and rectilinear grating 3c shifted by π relative to each other. Light transmitted through the transparent substrate 3 (transmitted light) is diffracted by the rectilinear grating 3b and rectilinear grating 3c, with ±1st order diffracted light beams generated in addition to zero order diffracted light (light that is transmitted without change) (the grating-diffracted light is hereinafter referred to as “Gr-diffracted light”). Since the wavefront of the zero order Gr-diffracted light is not affected by the grating, no phase change takes place, but the wavefronts of the ±1st order Gr-diffracted light beams are phase-shifted by π between the left and right of the axis 3Y serving as a boundary. These Gr-diffracted light beams form light spots on the signal plane 8a of the optical disc 8. In addition, during tracking control, a light spot 2b, which corresponds to the zero order Gr-diffracted light, is positioned directly on a guide groove 8g. Each of light spots 2b′, 2b″, which correspond to the ±1st order Gr-diffracted light beams, becomes two light spots separated in the optical disc radial direction about the guide groove 8g. This phenomenon of the two light spots 2b′, 2b″ being doubled respectively is due to the fact that the wavefronts of the ±1st order Gr-diffracted light beams are phase-shifted by π between the left and right of the central axis 3Y serving as a boundary. It should be noted that the diffraction efficiencies of the rectilinear grating 3b and rectilinear grating 3c are set such that the quantities of light of the light spots 2b′, 2b″ are respectively approximately 1/10 of the quantity of light of the light spot 2b. 
FIG. 9(a) is a diagram illustrating the configuration of a light detection surface used in a conventional optical disc device and the distribution of light on the light detection surface. FIG. 9(b) is a diagram illustrating light fluxes prior to incidence upon the cylindrical lens used in the optical disc device. The light fluxes 2c, 2c′, and 2c″ prior to incidence upon the cylindrical lens 9 respectively correspond to the light spots 2b, 2b′, and 2b″ on the signal plane 8a of the optical disc 8. Diffracted light beams 2cp and 2cm, diffracted by the guide grooves 8g of the optical disc 8, are superimposed upon the zero order Gr-diffracted light 2c while being shifted along the axis 9X, which corresponds to the optical disc radial direction (the guide groove-diffracted light is hereinafter referred to as the “groove-diffracted light”). The zero order groove-diffracted light beams of the ±1st order Gr-diffracted light beams 2c′ and 2c″ are phase-shifted by π between the left and right of the axis 9Y serving as a boundary, which is parallel to the optical disc rotation direction, with the ±1st order groove-diffracted light beams superimposed upon the zero order groove-diffracted light beams while being π-shifted along the axis 9X. Light spots 2d, 2d′, and 2d″ on the light detection surface 10a respectively correspond to the light fluxes 2c, 2c′ and 2c″ prior to incidence upon the cylindrical lens 9. Since the distribution of the light fluxes 2c, 2c′ and 2c″ is inverted with respect to the central axis of the cylindrical surface of the cylindrical lens 9 as a result of passage through the cylindrical lens 9, the distribution of the light spots 2d, 2d′, and 2d″ on the light detection surface 10a is rotated as a whole through 90 degrees relative to the light fluxes 2c, 2c′, and 2c″ (not only the distribution of light, but also the direction of travel of the light spots during a lens shift of the objective lens 7 (hereinafter, a lens shift of the objective lens also is referred to simply as a “lens shift”) is rotated through 90 degrees as well). Light detectors 11, 11′ and 11″ are arranged on the light detection surface 10a so as to be substantially coaxial with the light spots 2d, 2d′, and 2d″. Each light detector 11, 11′ and 11″ is divided into four detection cells (detection cells 11a, 11b, 11c, 11d; detection cells 11a′, 11b′, 11c′, 11d′; and detection cells 11a″, 11b″, 11c″, 11d″, respectively) by straight lines parallel to the axes 9X and 9Y (the straight line parallel to the axis 9Y is designated as 10X), with the points of intersection of the parting lines substantially coinciding with the centers of the light spots 2d, 2d′, and 2d″. 
In FIG. 9, the following twelve signals (detection signals) are obtained by the detection cells.    T1=signal obtained in detection cell 11a.     T2=signal obtained in detection cell 11b.     T3=signal obtained in detection cell 11c.     T4=signal obtained in detection cell 11d.     T1′=signal obtained in detection cell 11a′.     T2′=signal obtained in detection cell 11b′.     T3′=signal obtained in detection cell 11c′.     T4′=signal obtained in detection cell 11d′.     T1″=signal obtained in detection cell 11a′.     T2″=signal obtained in detection cell 11b″.     T3″=signal obtained in detection cell 11c″.     T4″=signal obtained in detection cell 11d″. Based on the following formulae (1)-(3), these detection signals are used to generate a tracking error signal TE associated with the tracks of the optical disc, a focus error signal FE associated with the signal plane of the optical disc, and a reproduction signal RF of the signal plane of the optical disc.
                    TE        =                              T            ⁢                                                  ⁢            1                    +                      T            ⁢                                                  ⁢            2                    -                      T            ⁢                                                  ⁢            3                    -                      T            ⁢                                                  ⁢            4                    -                      k            ×                          (                                                T                  ⁢                                                                          ⁢                                      1                    ′                                                  +                                  T                  ⁢                                                                          ⁢                                      2                    ′                                                  -                                  T                  ⁢                                                                          ⁢                                      3                    ′                                                  -                                  T                  ⁢                                                                          ⁢                                      4                    ′                                                  +                                  T                  ⁢                                                                          ⁢                                      1                    ″                                                  +                                  T                  ⁢                                                                          ⁢                                      2                    ″                                                  -                                  T                  ⁢                                                                          ⁢                                      3                    ″                                                  -                                  T                  ⁢                                                                          ⁢                                      4                    ″                                                              )                                                          Formula        ⁢                                  ⁢                  (          1          )                                        FE        =                              T            ⁢                                                  ⁢            1                    +                      T            ⁢                                                  ⁢            3                    -                      T            ⁢                                                  ⁢            2                    -                      T            ⁢                                                  ⁢            4                                              Formula        ⁢                                                  ⁢                                                (        2        )                                RF        =                              T            ⁢                                                  ⁢            1                    +                      T            ⁢                                                  ⁢            2                    +                      T            ⁢                                                  ⁢            3                    +                      T            ⁢                                                  ⁢            4                                              Formula        ⁢                                  ⁢                  (          3          )                    Here, the magnitude of the coefficient k is set so as to cancel lens shift-induced tracking error signal offsets generated during tracking control. For instance, when the quantity of light of the light spots 2d′ and 2d″ is approximately 1/10 of the quantity of light of the light spot 2d, the magnitude of the coefficient k is about 5.
FIG. 10, which is a diagram of a conventional optical disc device, is used to explain the lens shift-induced tracking error signal offset. Hereinafter the explanation will be based on a state before the 90-degree rotation by the cylindrical lens.
As shown in FIG. 10, when the objective lens 7 is shifted by ε from the optical axis L in the optical disc radial direction (in the direction of the axis 10X), Gauss-distributed light 2A, which is incident upon the objective lens 7 in a rotationally symmetric manner parallel to the optical axis L, is reflected from the signal plane 8a of the optical disc 8, thereby producing light 2B, whose distribution is shifted by 2ε (shifted only by ε relative to the central axis 7c of the objective lens 7). Accordingly, the distribution of the light spot 2d on the light detector 11 is centered on location 2D, which is shifted by an amount proportionate to 2ε (strictly speaking, by a value that is a product of 2ε and (half of astigmatic difference of cylindrical lens 9)/(focal length of collimating lens 5)), and the outside of the circle 7a (shifted by an amount proportionate to ε), which is a pattern produced by projecting the aperture of the objective lens 7 onto the light detection surface 10a along the light beam, is shielded from light. As a result, the quantity of light detected in the detection cells 11c, 11d exceeds the quantity of light detected in the detection cells 11a, 11b and an offset is generated in the tracking error signal obtained in the light detector 11 alone (TE=T1+T2−T3−T4). Offset generation in the light detectors 11′, 11″ is also completely identical to the offset generation in the light detector 11, such that, when normalized by the detected quantity of light, the same amount of offset with the same polarity is obtained for the tracking error signal obtained in the light detector 11′ alone (TE=T1′+T2′−T3′−T4′) and for the tracking error signal obtained in the light detector 11″ alone (TE=T1″+T2″−T3″−T4″). On the other hand, as concerns tracking error signals generated under off-track conditions in the light spots on the signal plane 8a of the optical disc 8, the tracking error signals obtained in the light detector 11 alone and the tracking error signal obtained in the light detectors 11′, 11″ alone have mutually opposed polarities. This is due to the fact that the interference relationship between the groove-diffracted light beams is inverted because the phases of the groove-diffracted light beams of the light spots on the light detectors 11′, 11″ are shifted by π between the left and right of the axis serving as a boundary, which is parallel to the optical disc rotation direction. Accordingly, unlike the tracking error signal (T1+T2−T3−T4) obtained in the light detector 11 alone, the tracking error signal obtained in accordance with the formula (1) makes it possible to cancel the influence of lens shift-induced off-track without affecting the sensitivity of detection (and even increase the detection output).
Patent document 1 JP H9-81942 A